One of the most famous (infamous?) anime underwear tropes for female characters is light blue and white striped underpants. Male characters, it seems, have a similar trope. [Image: An Di]
For the female characters, the stripes are horizontal, but for the male characters, they are vertical. As you can see below in this Twitter by Japanese Twitter user Takumi, a wide range of characters (from Lupin III to Ash from Pokemon) have all worn similar boxers.
— TAKUMI™ (@takumitoxin) May 15, 2016
(Apparently, Nick in the Disney feature film Zootopia does, too? I haven't seen it yet, but according to Takumi, this shows how influential the trope is. Though, Disney might not be directly influenced by this anime trend.)
Be aware that some of these characters also wear different coloured boxers. Ash, for example, has also worn white undies, while Lupin has worn heart boxers on several occasions.
Striped, however, are most common for the famous thief.
But who started this striped boxer trope in Japan? Some wondered if it wasn't early 1970s manga Otoko Oidon:
— 粉雪 (@conayuki) May 16, 2016
お 1971年。これか？ RT pic.twitter.com/5lOE04mwQ7
— TAKUMI™ (@takumitoxin) May 16, 2016
Or the earlier Osomatsu-kun, which ran in the 1960s:
— 麗しの二重反転 (@1959kazuhiro39) May 16, 2016
It's possible that this type of underpants was popular at the time in Japan, or that artists simply needed a shorthand to easily express that a character was in his undies and not wearing shorts or swimwear.